Rechargeable electric storage devices have recently been adopted as the power sources of vehicles (e.g., an automobile and a motorcycle) and various devices (e.g., a portable terminal and a laptop personal computer). Examples of the electric storage devices include battery cells (e.g., a lithium-ion battery cell and a nickel-metal-hydride battery cell), and capacitors (e.g., an electric double layer capacitor). Various types of cells are provided. As one of such cells, there is available a cell which is provided with an electrode assembly in which a positive electrode and a negative electrode in the shape of a sheet are wound with a separator sandwiched therebetween and a current collector connected to this electrode assembly.
As shown in FIG. 9C, a positive electrode 21, a negative electrode 22 and a separator 23 are stacked in the order of the separator 23, the negative electrode 22, the separator 23 and the positive electrode 21. These are wound in cylindrical shape to have the positive electrode 21 located outside. After that, the side surfaces of the cylindrical shape are pressed from both sides, whereby the side surfaces are crushed into a flat shape and deformed. Thus, the electrode assembly is fabricated. Alternatively, the electrode assembly is fabricated such that a laminate of the separator 23, the negative electrode 22, the separator 23, and the positive electrode 21, which are stacked in this order from the inner side, is wound into a flat shape.
As shown in FIG. 9A, the positive electrode 21 is provided with a positive-electrode active material layer (a positive-electrode active material coated portion) 21b on each of both surfaces of a positive-electrode current collector substrate 21a. This positive-electrode active material layer is formed, for example, by applying a positive-electrode active material paste to one surface of the positive-electrode current collector substrate 21a, drying the paste, then similarly applying a positive-electrode active material paste to the other surface of the positive-electrode current collector substrate 21a and drying the paste. The positive-electrode current collector substrate 21a is formed from, for example, long strip-shaped aluminum foil.
As shown in FIG. 9B, the negative electrode 22 is provided with a negative-electrode active material layer (a negative-electrode active material coated portion) 22b on each of both surfaces of a negative-electrode current collector substrate 22a. This negative-electrode active material layer is formed, for example, by applying a negative-electrode active material paste to one surface of the negative-electrode current collector substrate 22a, drying the paste, then similarly applying a negative-electrode active material paste to the other surface of the negative-electrode current collector substrate 22a and drying the paste. The negative-electrode current collector substrate 22a is formed from, for example, long strip-shaped copper foil.
More specifically, with the exception of one end portion of the positive-electrode current collector substrate 21a in the width direction, the positive electrode 21 is, for example, coated with a positive-electrode active material paste on both surfaces. Whereby, the positive electrode 21 is provided with the positive-electrode active material layer 21b on each of both surfaces of the positive-electrode current collector substrate 21a except this end portion. For this reason, in this end portion, the positive-electrode current collector substrate 21a (a positive-electrode active material layer-non-formed portion 21c) is exposed. On the other hand, with the exception of one end portion of the negative-electrode current collector substrate 22a in the width direction, the negative electrode 22 is, for example, coated with a negative-electrode active material paste on both surfaces. Whereby, the negative electrode 22 is provided with the negative-electrode active material layer 22b on each of both surfaces of the negative-electrode current collector substrate 22a except this end portion. For this reason, in this end portion, the negative-electrode current collector substrate 22a (a negative-electrode active material layer-non-formed portion 22c) is exposed.
As shown in FIG. 9C, the separator 23 physically isolates the positive electrode 21 and the negative electrode 22 from each other and holds an electrolyte.
In order to prevent the precipitation of dendrite and the like, the negative-electrode active material layer 22b is applied with a larger width than the positive-electrode active material layer 21b. The separator 23, which provides insulation between the positive electrode 21 and the negative electrode 22, has a larger width than the positive-electrode active material layer 21b and the negative-electrode active material layer 22b. However, the separator 23 has a width not covering the positive-electrode active material layer-non-formed portion 21c or the negative-electrode active material layer-non-formed portion 22c, which protrude widthwise.
As shown in the conceptual diagram of FIG. 10, an electrode assembly 20 (a wound electrode assembly 80) of a cell described in Patent Literature 1 (Japanese Patent Application Laid-Open No. 2010-287513) is such that an inner-circumferential end portion 22d (indicated by a solid line) of the negative electrode 22 covers an inner-circumferential end portion 21d (indicated by a solid line) of the positive electrode 21 and is wound so that the negative electrode 22 is disposed on an innermost circumference of the electrode assembly 20. As shown in the conceptual diagram of FIG. 11, an electrode assembly 20 (an electrode assembly group 2) of a cell described in Patent Literature 2 (Japanese Patent Application Laid-Open No. 2008-251256) is such that an inner-circumferential end portion 21d (indicated by a solid line) of the positive electrode 21 covers an inner-circumferential end portion 22d (indicated by a solid line) of the negative electrode 22 and is wound so that the positive electrode 21 is disposed on an innermost circumference of the electrode assembly 20. In FIGS. 10 and 11, the number of windings of the positive electrode 21 and the negative electrode 22 is smaller than it really is in order to make the configuration of winding clearly understandable. In an actual electrode assembly 20, winding is performed in a larger number and in a denser manner. In FIGS. 10 and 11, the illustration of the separator 23 is omitted.
Each of the electrode assemblies 20, 20 is of a flat shape. The electrode assembly 20 has first and second flat portions 20a, 20a, and first and second curved portions 20b, 20b. The first and second flat portions 20a, 20a are opposed to each other. The first and second curved portions 20b, 20b connect end portions of the first and second flat portions 20a, 20a together. The electrode assembly 20 described in Patent Literature 1 is wound in such a manner that a leading-end edge of the inner-circumferential end portion 22d of the negative electrode 22 is positioned in the flat portion 20a. The electrode assembly 20 described in Patent Literature 2 is wound in such a manner that a leading-end edge of the inner-circumferential end portion 21d of the positive electrode 21 is positioned in the flat portion 20a. That is, in each electrode assembly 20, the leading-end edge of the inner-circumferential end portion of the electrode disposed on the innermost circumference is positioned in the flat portion 20a. 